Cobalt-cemented tungsten carbide is a composite material that is widely used to make metal working tools, particularly metal cutting tools. Other metal working tools, such as milling cutters, reamers, drillers, etc., are made of metal alloys such as high speed steel.
One disadvantage of cobalt-cemented tungsten carbide is that it has a relatively soft matrix of binder phase material. Many have attempted to harden the surface of cobalt cemented tungsten carbide tools by coating the working surfaces with a protective layer of harder material, such as diamond or amorphous diamond-like carbon (DLC). Unfortunately, DLC coatings do not adhere well to cobalt-cemented tungsten carbide.
One reason for the poor adherence of DLC to cobalt-cemented tungsten carbide is the fact that cobalt comprises between about 5-15%--typically about 6%--of the composite. The cobalt acts as an adhesive to wet the carbide grains. The cobalt also has a catalytic effect upon the formation of graphite--a weak material. As a result, a preponderance of graphite and weak sp.sup.2 bonds are formed in the coating instead of strong, three-dimensionally bonded "diamond-like" carbon.
Researchers have attempted to prevent the formation of graphite instead of DLC by removing cobalt from the surface of cobalt-cemented tungsten carbide using acid etching, plasma etching, and other methods. Unfortunately, etching reduces the necessary support for the carbide grains, leaving them susceptible to removal under stress. In an attempt to overcome this result, copper has been electroplated onto the substrate to fill voids left by the etching. Unfortunately, none of the methods currently used to promote adherence of DLC to cobalt-cemented tungsten-carbide components has been entirely successful.
DLC coatings also are difficult to adhere to the surfaces of metal alloy working tools made of high speed steel. Originally, DLC coatings were formed on metal alloys using chemical vapor deposition, which required that the workpiece or surface be subjected to a temperature of about 750.degree. C. The only metal alloys that can be treated using such high temperatures are not strong in tension. Also, a continuous diamond-like carbon film has an exceptionally low coefficient of thermal expansion and an extremely high modulus of elasticity. No commercially available metal alloys could both tolerate the high temperatures required to form a diamond-like carbon coating using CVD, and provide sufficient mechanical support for a continuous diamond-like carbon coating, even under the significant internal stresses which develop during metal cutting and finishing.
The use of ion beam assisted deposition to deposit DLC coatings overcomes the need to use high temperatures during the deposition process. DLC that is deposited using ion beam assisted deposition has far lower residual stress than chemical vapor deposited DLC, and is a better candidate for a high integrity DLC. However, it is difficult to form a DLC coating which will adhere strongly enough to the surface of any metal working tool that the DLC will resist cracking, loss of adhesion, and ultimate spallation.
Some have suggested forming an intermediate "bondcoat" to more strongly adhere the DLC coating to the cutting surface. The substrate material to which all forms of carbon adhere most successfully is silicon. This is because strong covalent Si--C bonds are easily formed between the coating and the silicon substrate. Some have attempted to improve the adhesion of DLC to metal alloys by forming an interposed silicon bond-coat to which the DLC will adhere more strongly. Unfortunately, this simple approach does not result in adhesion that survives in applications where the DLC coating is subjected to substantial friction and stress. The simple formation of a silicon bond-coat on a metal alloy appears to create another relatively weak interface between the silicon and the metal alloy.
Methods are needed to produce metal working tools with cutting surfaces bearing DLC coatings that are bound to the surface strongly enough to resist cracking, loss of adhesion, and ultimate spallation.